From: Brian Candler <b.candler@...>
Date: 2010-03-31T04:35:43+09:00
Subject: Re: require and include??

jbw wrote:
> This might clear things up, but to summarise  include adds methods,
> require adds modules.

That is almost completely inaccurate, I'm afraid.

'require' loads and executes code. For example,

    require "foo"

looks for foo.rb or foo.so in the $LOAD_PATH and then loads and executes 
it. (Actually, it first looks in $LOADED_FEATURES to see if it has 
already been loaded, and passes if it has. Use 'load' to load and 
execute unconditionally)

foo.rb *might* contain code which defines modules and/or classes, but it 
doesn't have to. require doesn't care, it just executes whatever is in 
there.

$ cat foo.rb
puts "hello world"

$ irb --simple-prompt
>> require 'foo'
hello world
=> true
>> require 'foo'
=> false                # has already been loaded
>> 


'include' is used to add the instance methods of a module into a class.

$ irb --simple-prompt
>> module Bar; def hello; puts "hello world"; end; end
=> nil
>> class Foo; include Bar; end
=> Foo
>> Foo.new.hello
hello world
=> nil
>> 

It also works at the 'top level', which is probably what you're thinking 
of.

>> include Math
=> Object
>> sqrt(9)         # short for Math.sqrt(9)
=> 3.0

Notice that 'include' takes a Module as its argument, whereas 'require' 
takes a String (which is the filename to load).
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